It is essential for normal breathing that the space within the pleural cavity surrounding the lungs be free of liquid and be subject to a negative pressure so as to draw the lungs outwardly to fill this pleural cavity in order to permit proper breathing. Any invasion of the pleural cavity such as caused by lung surgery or foreign objects which pierce the rib cage or such as occur, for example, where the patient has pleurisy, generates fluids in the pleural cavity which tend to obstruct normal breathing by preventing full expansion of the lungs. It is necessary to provide a device which can remove these fluids from the pleural cavity and at the same time ensure that the desired degree of negative pressure imposed by the suction control chamber is approximately maintained within the pleural cavity so that the lung maintains its maximal expansion.
Two of the basic types of apparatus which have been used for this purpose are shown, for example, in U.S. Pat. Nos. 3,363,626 and 3,363,627, and in pending U.S. application Ser. No. 120,295 filed Feb. 11, 1980, which are herein incorporated by reference. The first of these apparatuses provides three chambers, one chamber comprising a collection chamber for collecting the fluids drained from the pleural cavity through a thoracotomy tube, a second chamber known as an underwater seal chamber which protects the pleural cavity from being subject to atmospheric pressure, and a third chamber known as a suction control chamber which serves to regulate the degree of negative pressure within the pleural cavity. The other of these apparatuses provides a collection chamber with an underwater seal chamber located at the upper end thereof adjacent the lower end of the thoracotomy tube. Secretions from the body cavity form the underwater seal and excess secretions thereafter overflow into the collection chamber. These types of apparatuses have been highly successful in both removing fluids from the pleural cavity and in maintaining the desired degree of negativity within the pleural cavity.
It has been found that nurses frequently will "milk" the thoracotomy tubes in an effort to remove any clots or obstructions from the tube. This "milking" of the tube is achieved by squeezing the flexible thoracotomy tube adjacent the upper end and drawing the fingers down the tube to cause the fluids within the tube to be passed out the lower end of the tube and into the collection chamber. Obviously, this action has the effect of substantially lowering the degree of negativity within the pleural cavity. Such high negativity can be damaging to the pleural cavity and may also cause the liquid within a combined water seal-collection chamber to be drawn up into the pleural cavity. In addition, the entire water seal can be lost into the pleural space or the collection chamber during periods of high negativity in the pleural cavity. The loss of the water seal has the potential for causing pneumothorax in the event that the suction becomes disconnected. Thus, there is need for a means of providing necessary relief for the condition of excess negativity in the pleural cavity.
In one of the applicant's pending U.S. application Ser. No. 256,152, a metered air pump is disclosed by which excess negative pressure can be relieved by pumping as many small units of air into the thoracotomy tube as necessary. In another of applicant's pending U.S. application Ser. No. 309,796, an automatically operated valve connects the thoracotomy tube directly with the atmosphere whenever excess negativity occurs.
When excess negativity is relieved in the collection chamber and/or the thoracotomy tube, it is important that the pressure in the collection chamber and/or the thoracotomy tube be prevented from reaching atmospheric pressure. Should the pressure in the collection chamber and/or the thoracotomy tube reach atmospheric pressure, the collection chamber immediately ceases to drain fluids from the pleural cavity, a pneumothorax develops, the lung collapses and breathing of the patient can quickly become difficult. So long as the collection chamber and/or the thoracotomy tube are subatmospheric, a pneumothorax does not occur.
Even though drainage devices have been developed which do not require a filling of the underwater seal chamber (see also, for example, U.S. Pat. Nos. 4,015,603 and 4,312,351), these devices generally do not provide a direct indication of the suction force being exerted. Such a feature is, of course, highly desirable in a drainage device.